The present invention relates to a process and apparatus for cryogenically separating air, and in particular to such a process and apparatus for the production of nitrogen.
It is well known to supply part of the refrigeration requirements of an air separation process by liquid assist. This involves the injection of a cryogenic liquid into the distillation column or a condenser of the column at a point where the liquid in the column has a similar composition to that of the liquid used for the liquid assist.
Examples of liquid assist processes are to be found in U.S. Pat. No. 2,908,144 and U.S. Pat. No. 3,620,032 and in &lt;&lt;Recent Developments in Industrial Oxygen Production &gt;&gt; by M. P. Dubs, Trans. Instr. Chem. Engs. Vol. 36, 1958.
The standard technique used to regulate the amount of liquid assist sent to a single column is to vary the amount of liquid injected into the column in dependence on the level of liquid in the top condenser of the column (see for example EP-A-0 144 430, EP-A-191 862, J53-14351, J61-24968, U.S. Pat. No. 2,685,181). The same regulation technique is disclosed in a general context in FR 2 076 020.
It is an object of the present invention to provide an air separation process which can be more accurately controlled.
According to the invention, there is provided a cryogenic air separation process in a distillation column having a top condenser, comprising compressing and purifying the air; cooling the air after compression and purification thereof to a temperature suitable for distillation;
separating the air in a distillation column so that an oxygen enriched liquid and a nitrogen enriched gas are produced within the column; sending a stream of cryogenic liquid to the column; PA1 sending oxygen-enriched liquid from the column to the condenser; PA1 regulating a flow rate of the cryogenic liquid in dependence on the level of oxygen enriched liquid at the bottom of the column; and PA1 withdrawing a product stream from the column. PA1 a distillation column having a top condenser, PA1 means for providing cooled compressed air to the distillation column, PA1 means for removing nitrogen enriched fluid from the column, PA1 means for sensing a liquid level at the bottom of the column, PA1 means for sending oxygen-enriched liquid from the bottom of the column to the condenser, PA1 means for sending a cryogen liquid to the column, PA1 means for controlling the flow rate of the cryogenic liquid sent to the column in dependence on said liquid level.
Preferably the column is single column nitrogen generator with a top condenser.
There is further provided according to the invention a cryogenic air separation apparatus comprising
The column may contain trays or packings of this structured or random type. As the column is generally of smaller diameter than the condenser, a requirement for increased refrigeration will be reflected by a drop in liquid level of greater magnitude that the drop in liquid level in the condenser. This enables the refrigeration requirement to met with greater accuracy.